Extending the life of the catalysts that are used for olefin oligomerisation, particularly, oligomerisation of C3 to C6 olefins, particularly, C3 and C4 olefins, in order to produce C8 and C10 olefins is highly desirable. The oligomerisation product may further be hydroformylated and hydrogenated to produce alcohols. C9, C10 and C12 alcohols are useful in many areas one of which is in esterification to produce plasticizer esters such as phthalates, sebaccates, cyclohexanoates, pyromellitates, and adipates. These materials are used to plasticise plastics such as polyvinyl chloride (PVC).
In all these oligomerisation processes regardless of the length of the run, the zeolite catalyst becomes deactivated over time usually due to the formation of deposits of carbon (known as coke) in the pores of the zeolite. This leads to a reduction in conversion of olefins to oligomers and a reduction in the selectivity of the conversion.
The H zeolites, e.g., ZSM-22 and ZSM-57, are preferred catalysts for the oligomerisation of olefins in that they have a pore structure which enables the production of olefin oligomers, particularly, octenes and nonenes having a structure suitable for hydroformylation to produce alcohols for plasticizer production. These catalysts are particularly prone to coke formation and thus require regeneration; however, they are also easily damaged under typical regeneration conditions.
There is therefore a need to optimise the regeneration of such catalysts in order to extend their life in the oligomerisation of olefins to produce feedstocks for hydroformylation to produce alcohols useful in the manufacture of plasticisers.
The coke that forms on these catalysts when they are used for olefin oligomerisation takes different forms and can be made up of light coke, heavy coke, and hard coke. Traditionally, coke is removed from catalysts by burning. However, the pore structure of ZSM-22 and ZSM-57 catalysts are susceptible to damage at the elevated temperatures that have traditionally been required to remove all the coke. For example, it has been necessary to remove hard coke from ZSM-22 by heating to temperatures around 600° C. which has been found to damage the zeolite structure. Damage to the structure starts to occur at about 510° C. and reduces the catalyst activity and selectivity in converting olefins to oligomers.
It has now been found that by control of the heating cycle and the amount of water present during the catalyst regeneration a substantial amount of the hard coke can be removed while reducing the amount of damage caused to the pore structure of the zeolite. It has also been found that it can be beneficial to retain a small amount of coke on the regenerated catalyst as a totally clean regenerated catalyst can be too delicate for subsequent oligomerisation operations.